Overview of project before I get to the question:
- Walls are up for an 8x16 root/storm cellar into the side of a hill. My top course is made up of L-block (or shoe block, or header block ... goes by many names). Vertical rebar every 4' & at corners, to be poured monolithically w/ the roof; ladder reinf every other course.

- Getting ready to form up for the roof pour ... looking at a 6" slab over 22ga corrugated steel deck; deck has 2" ribs. Don't worry ... I'll be using plenty of #4 rebar, 5K psi crete, & plenty of temp vertical supports in preparation for the pour.

Here's the question ... to get the 6" depth I want, I need to go 2" above the top of the L-block (2" + 4" to the web that the steel deck sits on). Putting up forms on the block exterior is no problem, but will there be good adherence between fresh concrete & block? Is 2" adequate cover over that block's 1.25" web? Or, should I put an expansion joint over that web?

SeniorSitizen

10-30-2012 10:47 AM

A drawing or even a rough sketch of the cross section would help visualize this project but if I'm seeing this correctly there will be a 90° corner ( 2+4 ) and that's where concrete is surely to crack.

jomama45

10-30-2012 01:10 PM

The easiest option that I can think of would be to simply cut/grind the outer shell of the block at about a 45 degree angle or steeper..........

avodahfarm

10-30-2012 01:40 PM

Thanks for the responses. Maybe I can get a pic or sketch later ... currently thawing my hands out - I can barely type!

I understand the thought about grinding the outside edge - is that to ensure better adherence? Would painting a bonding agent to the block just prior to pouring be helpful as well?

The header block I'm using is often used for suspended porches (somewhat similar to my project), so I'm hoping the block's inherent 90-degree corner will be OK. The load is being carried primarily by the rebar & down through the vertical columns ... i.e. that should be very little load on the outside web of the block. I'm also adding fiber to the concrete mix, so maybe that will help with cracking concerns?

jomama45

10-30-2012 01:46 PM

I understand completely what you're doing I believe, I think you posted a few pictures on the topic a few weeks back. I should have explained better, I'd suggest grinding/cutting the inside of the outer web, not the outside. WIll it crack otherwise? Possibly, but unless it's going to be exposed as a finished product, it shouldn't matter much. ANd, If I were you, I'd skip the fibers as they're offer nothing once the concrete is cured.........